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The Effect of Mount St. Helens' Ash on Soil Microbial Respiration and Numbers¹

ABSTRACT

Incubation studies were conducted to determine the effect of different levels of Mount St. Helens' ash on soil microbial activity. Ritzville silt loam was mixed with 1% wheat straw (Triticum aestivum L.) and amended with 0, 10, or 20% ash or 10 or 20% sand. Oxygen uptake by the soil-sand mixes was significantly greater than by the other treatments, and there was a trend for greater CO₂ evolution by the sand-amended treatments. These differences likely resulted from an aeration effect and/or differences in the systems caused by the mixtures, and not from salinity or release of toxic elements from the ash. Total bacterial, fungal, and pseudomonad counts in the soil were unaffected by either ash or sand additions with or without a wheat straw amendment during a 20-day incubation period. These results show that the ash additions to the soil from the volcanic eruption should not adversely affect crop residue decomposition or the microbial flora of the soil as measured by soil microbial activity and numbers.

Key Words: wheat straw decomposition • O₂ uptake • CO₂ evolution • toxic elements • soil microbial activity

¹ Contribution of USDA-ARS, in cooperation with the College of Agric. Res. Ctr., Washington State Univ., Pullman, WA 99164. Sci. paper no. 5920.

² Microbiologist, Research Biologist, Soil Scientist, and Soil Scientist, respectively, USDA-ARS; and Soil Scientist, Washington State Univ.

Received for publication July 27, 1981.